178
R. A. Ribeiro Filho et al.
which only 10 % refer to bed material load. The suspended
load varies according to the flood peaks (precipitation).
The turbidity of water the measurement that expresses the
loss of light penetration into the aquatic environment, as it is
a result of all organic and inorganic material that is in suspension in water in the form of particles (Wetzel 2001). In
the Itaipu Reservoir this variable showed very similar values
among the riverine, transition and lacustrine zones, although
it presented declining average values in the upstream-downstream direction, confirming the results of Andrade et al.
(1988). The highest values of turbidity can be explained by
occasional periods of rain just before and during the collection of data, as observed by Zanata (1999) in Salto Grande
reservoir (Americana, SP) and by Pinto Coelho et al. (2005)
in São Simão Reservoir (MG). This variable is very important because many species of predator fishes have a visual
active persecution feeding tactics (Ribeiro filho 1999), promoting the resuspension of organic and inorganic materials
as they move around to catch food (Pedro 1995). This variable is directly related to the decrease in water flow, resulting
in a lower input of suspended materials in the water.
According to Esteves (1998), the pH represents the ionic
balance in the aquatic environment, interfering directly with
the conditions of neutrality of water and environmental buffering capacity (alkalinity). The increase of this variable may
be related to changes in the values of alkalinity in aquatic
ecosystems, explaining the inverse relationships of the alkalinity values found with regard to pH. The Itaipu Reservoir,
in general, presented an alkaline pH with slight tendency
to neutrality. These results are in consonance with those by
Agostinho et al. (1997a). The fact that the transition zone
presents higher pH values may be related to higher photosynthetic activity (cyanobacteria) in this region (Vollenweider 1974). Shapiro and Wright (1984), in an experimental
study, found that concentrations of pH and CO 2 are variables
that help to determine the cyanobacteria populations.
Electrical conductivity is the resistivity ability to transmit
electricity in the water body, reflecting indirectly the level
of dissolved solids in water. In some studies the increase in
conductivity was accompanied by the increase of cyanobacteria (Ramirez 1996). In the Itaipu Reservoir, although quite
discrete, the highest conductivity was recorded in the transition zone. The exclusive presence of tilapia ( Oreochromis
niloticus and Tilapia rendalli) when compared to treatments
without the presence of fishes in studies by Starling and
Rocha (1990) and Figueredo (2000), showed an increase
in the values of electrical conductivity as a result. The ion
concentrations in the lacustrine area can be explained by the
higher biomass of piscivore species, justified by the control
of omnivore biomass (Ribeiro Filho 2002).
The dissolved oxygen in the aquatic ecosystem is a limiting factor for both nutrient cycling and for primary productivity (Agostinho et al. 1997a). In this study, we observed a
pattern in their distribution with regard to seasonality during
the months the data were collected, confirming the results
reported by Agostinho et al. (1999), in which the lowest dissolved oxygen concentrations were found in summer (January to March) and the highest during the rainy season (June
to August). For this work, the highest amount of dissolved
oxygen in the transition zone may be associated with the
great development of algae, especially cyanobacteria. The
month of March, for all years, presented the lowest concentration of this gas, as it is strongly affected by water temperature (in warmer months there is loss of oxygen into the
atmosphere and increased metabolism of the environment).
In a study by Minillo (2005) in reservoirs in the middle
and lower Tietê River (SP) high concentrations of dissolved
oxygen were found in the water surface. The author stresses
that these findings may be related to the rainy months and
other studies also emphasize the effect of rainfall in the distribution pattern of dissolved oxygen (Townsend 1999).
The BOD variable represents the amount of oxygen required to oxidize organic matter by microbial decomposition. The increase in the values of this variable represents an
increase of organic or punctual pollution (CETESB 2006).
The transition zone showed the highest mean values owing
to swine breeding activity in that area of the watershed (Benassi, personal communication, 2006).
The COD variable represents the amount of oxygen required to oxidize organic matter by means of chemical
agents. The increases in the DQO values in water bodies are
indicative of industrial pollution (CETESB 2006). As with
the BOD, COD showed a higher average concentration in
the transition zone (7.5 mg/L), which can also be associated
with the input of materials into the reservoir.
The nitrogen and phosphorus are the main nutrients for
aquatic environment primary producers (Esteves 1988) and
the mean increase of nutrients means changes in the abundance of fishes, in what regards composition and biomass
in temperate lakes (McQueen 1998). The fish biomass and
their density per unit area of the lake increase with increasing concentration of total phosphorus (Hanson and Lagget
1982; Quiros 1990), so that the concentration of predatory
fishes is reduced (Jeppesen et al. 1990; Persson et al. 1988).
Among the nitrogen forms mostly consumed by the communities of aquatic ecosystems are ammonia nitrogen, nitrate, and nitrite. Nitrite is the intermediate state between
ammonia and nitrate, during the nitrification process, and is
not very much available in the environment and used by primary producers. Nitrite is a very complex variable, because
it is converted into ammonia or nitrate and unlikely to accumulate in the system, what explains the lack of relationship
with chlorophyll-a. Their low concentrations may also indicate a high oxygen environment (Esteves 1988).
The nitrate concentrations in the reservoir had a horizontal longitudinal distribution. The results were similar to those
R. A. Ribeiro Filho et al.
which only 10 % refer to bed material load. The suspended
load varies according to the flood peaks (precipitation).
The turbidity of water the measurement that expresses the
loss of light penetration into the aquatic environment, as it is
a result of all organic and inorganic material that is in suspension in water in the form of particles (Wetzel 2001). In
the Itaipu Reservoir this variable showed very similar values
among the riverine, transition and lacustrine zones, although
it presented declining average values in the upstream-downstream direction, confirming the results of Andrade et al.
(1988). The highest values of turbidity can be explained by
occasional periods of rain just before and during the collection of data, as observed by Zanata (1999) in Salto Grande
reservoir (Americana, SP) and by Pinto Coelho et al. (2005)
in São Simão Reservoir (MG). This variable is very important because many species of predator fishes have a visual
active persecution feeding tactics (Ribeiro filho 1999), promoting the resuspension of organic and inorganic materials
as they move around to catch food (Pedro 1995). This variable is directly related to the decrease in water flow, resulting
in a lower input of suspended materials in the water.
According to Esteves (1998), the pH represents the ionic
balance in the aquatic environment, interfering directly with
the conditions of neutrality of water and environmental buffering capacity (alkalinity). The increase of this variable may
be related to changes in the values of alkalinity in aquatic
ecosystems, explaining the inverse relationships of the alkalinity values found with regard to pH. The Itaipu Reservoir,
in general, presented an alkaline pH with slight tendency
to neutrality. These results are in consonance with those by
Agostinho et al. (1997a). The fact that the transition zone
presents higher pH values may be related to higher photosynthetic activity (cyanobacteria) in this region (Vollenweider 1974). Shapiro and Wright (1984), in an experimental
study, found that concentrations of pH and CO 2 are variables
that help to determine the cyanobacteria populations.
Electrical conductivity is the resistivity ability to transmit
electricity in the water body, reflecting indirectly the level
of dissolved solids in water. In some studies the increase in
conductivity was accompanied by the increase of cyanobacteria (Ramirez 1996). In the Itaipu Reservoir, although quite
discrete, the highest conductivity was recorded in the transition zone. The exclusive presence of tilapia ( Oreochromis
niloticus and Tilapia rendalli) when compared to treatments
without the presence of fishes in studies by Starling and
Rocha (1990) and Figueredo (2000), showed an increase
in the values of electrical conductivity as a result. The ion
concentrations in the lacustrine area can be explained by the
higher biomass of piscivore species, justified by the control
of omnivore biomass (Ribeiro Filho 2002).
The dissolved oxygen in the aquatic ecosystem is a limiting factor for both nutrient cycling and for primary productivity (Agostinho et al. 1997a). In this study, we observed a
pattern in their distribution with regard to seasonality during
the months the data were collected, confirming the results
reported by Agostinho et al. (1999), in which the lowest dissolved oxygen concentrations were found in summer (January to March) and the highest during the rainy season (June
to August). For this work, the highest amount of dissolved
oxygen in the transition zone may be associated with the
great development of algae, especially cyanobacteria. The
month of March, for all years, presented the lowest concentration of this gas, as it is strongly affected by water temperature (in warmer months there is loss of oxygen into the
atmosphere and increased metabolism of the environment).
In a study by Minillo (2005) in reservoirs in the middle
and lower Tietê River (SP) high concentrations of dissolved
oxygen were found in the water surface. The author stresses
that these findings may be related to the rainy months and
other studies also emphasize the effect of rainfall in the distribution pattern of dissolved oxygen (Townsend 1999).
The BOD variable represents the amount of oxygen required to oxidize organic matter by microbial decomposition. The increase in the values of this variable represents an
increase of organic or punctual pollution (CETESB 2006).
The transition zone showed the highest mean values owing
to swine breeding activity in that area of the watershed (Benassi, personal communication, 2006).
The COD variable represents the amount of oxygen required to oxidize organic matter by means of chemical
agents. The increases in the DQO values in water bodies are
indicative of industrial pollution (CETESB 2006). As with
the BOD, COD showed a higher average concentration in
the transition zone (7.5 mg/L), which can also be associated
with the input of materials into the reservoir.
The nitrogen and phosphorus are the main nutrients for
aquatic environment primary producers (Esteves 1988) and
the mean increase of nutrients means changes in the abundance of fishes, in what regards composition and biomass
in temperate lakes (McQueen 1998). The fish biomass and
their density per unit area of the lake increase with increasing concentration of total phosphorus (Hanson and Lagget
1982; Quiros 1990), so that the concentration of predatory
fishes is reduced (Jeppesen et al. 1990; Persson et al. 1988).
Among the nitrogen forms mostly consumed by the communities of aquatic ecosystems are ammonia nitrogen, nitrate, and nitrite. Nitrite is the intermediate state between
ammonia and nitrate, during the nitrification process, and is
not very much available in the environment and used by primary producers. Nitrite is a very complex variable, because
it is converted into ammonia or nitrate and unlikely to accumulate in the system, what explains the lack of relationship
with chlorophyll-a. Their low concentrations may also indicate a high oxygen environment (Esteves 1988).
The nitrate concentrations in the reservoir had a horizontal longitudinal distribution. The results were similar to those